This invention relates to the development and synthesis of manganese activated. alkaline earth aluminate phosphor particles having a uniform particle size distribution (0.05 to 5 xcexcm) and a method of forming such particles. More specifically, this invention relates to a development and method of formation of such particles from respective oxides, nitrates and organic precursors which form small particles that improve the performance parameters of higher brightness, shorter persistence, better stability, longer life time and good color saturation as required for flat panel display (FPD) and lamp applications.
Manganese activated zinc silicate phosphor is currently used in plasma display panels (PDP) as a green emitting component due to its availability and high quantum efficiency. When compared with other phosphors used for red and blue, zinc silicate phosphor exhibit longer persistence, lower dielectric constant and faster saturation with the VUV flux. Efforts are being made to develop new phosphors to satisfying all requirements and replace manganese activated zinc silicate phosphor. Some other phosphor candidates based on alkaline earth aluminates are being suggested in Phosphor Handbook edited by S. Shionoya and W. M. Yen, pp 630-636, CRC Press, 1999. U.S. Pat. No. 4,085,351 discloses the application of manganese activated aluminate phosphor with either of calcium, strontium, barium, magnesium or zinc in a gaseous discharge light emitting element. In U.S. Pat. No. 5,868,963 M. P. Thi and A. L. Fur describe the preparation of manganese activated barium aluminate by calcinating the mixture of alumina, barium carbonate, manganese carbonate at 1450xc2x0 C. for two hours in dry nitrogen. T. Hisamune et al. in EP 0 908 502 A1 teach the preparation of barium or strontium magnesium aluminate by firing respective oxides or carbonate in presence of flux (AIF3) at 1450xc2x0 C. for 48 hours (total time). Mark H. Smith et al in WO 98/37165 describe a method of making oxygen containing phosphor powder, including alkaline earth aluminates by spray techniques. According to that invention, spray dried powder was converted to oxide at 1000xc2x0 C. and then re-fired at 1600xc2x0 C. for about four hours.
The main application of large area plasma displays will be HDTV and high information content presentation. HDTV and similar type of display devices should have phosphors with low dielectric constant, required decay time, high resolution and high brightness for high performance. This can be achieved only with thin phosphor screens consisting of small phosphor particles in a close rib structure or closed cell structure. Screens with small particles exhibit higher packing density and also need reduced binder content. Persistence, another concern in selecting a phosphor, should be between 4 and 9 ms. Also, the three phosphors (red, green and blue) currently used in PDP""s have different-dielectric constants and particle morphology. Due to their physical nature, all of the three phosphors need different rheology of phosphor paste as well as different screening processes. In PDP applications these phosphors exhibit different electrical characteristics in a finished panel. The higher dielectric constant of zinc silicate phosphor is of particular concern as it charges more than its blue and red counterparts and this results in a higher sustainer voltage. This results in compromises in the performance of the display. In consideration of these problems, we have dedicated our efforts to developing phosphors consisting of similar host materials, which exhibit suitable red, green and blue emission under Xenon plasma excitation.
Accordingly, it is an objective of the present invention to provide a method of preparation of manganese activated alkaline earth aluminate phosphor having the empirical formula:
(AE1-xMnx)Oxc2x76(AI2O3)
wherein AE is selected from Ba, Sr, Ca and Mg and 0.01xe2x89xa6xc3x97xe2x89xa60.1.
The present invention compares the synthesis of manganese activated alkaline earth aluminate phosphor by two different processes: a conventional solid state reaction process and our sol-gel process. Depending on the required particle size distribution, the sol-gel process is superior for preparing very fine particles (0.05 to 2 microns) and the solid state reaction is superior for normal size particles (2 to 6 microns).
The sols are dispersions of colloidal particles in a liquid. The gravitational forces on the particles are negligible. From a sol, a gel is formed with an interconnected, rigid network, having sub-micrometer pores and a polymeric chain whose average length is of the order of microns. The particle size of the finished product is a function of the initial concentration of the starting sols, gelation process, drying of gels, calcination temperature and rate of cooling.
The sol-gel process offers many advantages over conventional methods in the synthesis of fine powders and particularly phosphor materials. Since all of the starting materials are mixed at the molecular level in a solution, a high degree of homogeneity is achievable. Doping of impurities (activators/co-activators/sensitizers) through solutions is straightforward, easy and effective. The pores in properly dried gels are often extremely small and the components of a homogenous gel are intimately mixed. The surface area of powders produced from sol-gel is very high, leading to lower processing temperatures.
Phosphor materials are extremely sensitive to impurities; even in ppb levels, the low-temperature process through sol-gel process minimizes the potential for cross contamination. Some of the unwanted impurities left in the materials from conventional methods may pose a threat to the performance of a phosphor. As the size of the phosphor particle decreases, the probability of electron and hole capture to the impurity increases and the e-h localization enhances the recombination rate via the impurity. The optimum impurity concentration (activator) level can be further increased with small particle size. The present invention is related to the growth of Mn2+ doped alkaline earth aluminate phosphor by sol-gel methods.
More specifically, the present invention provides a process for forming a Mn2+ doped alkaline earth aluminate phosphor having the empirical formula:
(AE1-xMnx)Oxc2x76(Al2O3)
wherein AE is selected from Ba, Sr, Ca and Mg and 0.01xe2x89xa6xc3x97xe2x89xa60.1, the process comprising:
(1) reacting a dilute solution comprising a source of an alkaline earth, a source of manganese and an organic precursor providing a source of aluminum, in an acid medium to form a dilute gel (sol-gel process);
(2) converting the dilute gel into a xerogel powder (room temperature drying) or converting the dilute gel into an aerogel powder (vacuum drying); or converting the dilute gel into a gel powder (spray drying); and,
(3) thermally decomposing the powders obtained from the above, at specified temperatures.